Apparatus for measuring mass flow

ABSTRACT

The invention relates to an apparatus for weighing the mass flow of fluent material, especially of particulate type, by determining the forces exerted on a substantially in the direction of the flow rectilinear and inclined plate ( 3 ) on which the material ( 1 ) is sliding. The measuring plate ( 3 ) is fastened at one central level ( 8  - 8 ) of the plate, and the fastening device includes one moment measuring means ( 6 ) measuring the moment round a horizontal axis ( 7 ) which is crossing the vertical axis ( 8  -  8 ). The moment measuring means (6) is attached to a force measuring means ( 9 ) measuring the force directed in or parallel to the vertical axis ( 8  -  8 ). The force measuring means ( 9 ) is resting on or attached to the ground ( 10 ), and the material flow ( 1 ) is determined by a calculating means from the output signals from the moment measuring means ( 6 ) and the force/load measuring means ( 9 ).

DESCRIPTION

[0001] This invention relates to apparatus for weighing a continuousstream of fluent material.

[0002] There is a method of weighing a continuous stream of fluentmaterial which includes means to direct the material onto a weighingmeans which, in the direction of flow of the material, is arranged in aplane at an acute angle relative to a vertical plane and measuring, in aplane substantially parallel to the plane in which the weighing means isarranged, the load applied by the material to the weighing means andgenerating a signal representative of the mass of the material. Todetermine the mass flow, the velocity of the flow must be determined bya signal representative of the velocity. These two signals are thenprocessed in an electronic device where also the total material weighedis calculated.

BACKGROUND ART

[0003] Most of the number of weighing apparatuses of this and similartypes of practical reasons leave out the speed signal and the mass flowis to be determined by calibration with a known material flow. Anexample of equipment for producing a speed signal is disclosed in theU.S. Pat. No. 5,002,140, where a wheel with ribbed surface is placedwhere the material is leaving the weighing plate and can impinge tocause the wheel to rotate proportional to the speed of the material. Ofreliability reasons the method is limited to suitable materials and thespeed signal is not representing the mean value of the material slidingover the plate. It is also known from the European patent EP 0626062(U.S. Pat. No. 5,495,773) to calculate the mean velocity from thechanging of the flow profile when a particulate material is acceleratingalong the weighing plate. This is done by supporting the plate at twodifferent levels along the tilted plate by force measuring means. Athird force measuring means is also present to determine the frictionforce of the material against plate surface. The magnitude of the forcesexerted relative the capacity of the bulk flow is relatively small,where the number of supporting and pivot points in combination with thesmall order of forces attacking the measuring means make the suggestedconstructions delicate. Especially where small differences between twomeasured forces are to be measured.

SUMMARY OF THE INVENTION

[0004] It is the object of the present invention to provide an apparatusof the kind referred to, which gives a simple and sturdy mechanicalconstruction together with measuring of both the weight of the materialand its velocity.

[0005] A continuous flow of fluid or particulate material sliding on atilted plane increases its velocity by the gravitation force. Thesection of the stream thus decreases as the velocity increases. Fromthis behaviour the weight of the mass of the upper part will be largerthan of the lower part. Around a horizontal axis straight across thecentre part of the tilted plate a moment arises which is proportional tothe difference of the masses on each side. It can be shown that thedifference of the masses is proportional to the velocity of thematerial. By directly measuring that moment with a moment transducer onefactor for determining the velocity of the mass flow will be given. Theother factor is the sum of the weight of the material on the tiltedplate. That is done with a load transducer, which just is measuring theload in the direction of or parallel to one specified axis, and placedbelow the moment transducer, it measures the total load on the plateregardless of its distribution over the surface of the tilted plate. Theplate is attached in one central point on the underside of the plate oralong a horizontal axis and the load transducer is attached to theground carrying the plate via the moment transducer. It can be shownthat the moment signal in combination with the weight signal will give ameasurement of the flow. The background for the calculations is that therelative change in the height of the material sections in the upper andlower ends of the plate in comparison to the mean value of the height isproportional to the change of the velocity of the material flow incomparison with the mean velocity over the plate. Included in theinvention is the calculating means for indicating the flow.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is a schematic side elevation view of a flow meterembodying principles of the present invention; FIG. 2 is a larger scalefragmentary side elevational view of the centre specific embodimentthereof; and FIG. 3 is a schematic side elevation view of the completeflow meter and frame for smaller magnitudes of flows embodyingprinciples of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0007]FIG. 1 shows the basic design of the invention. The material 1 issliding along the inlet plate 2 from above with an angle φ and continuesover the measuring plate 3 and outlet plate 4. The measuring plate iscarried by an attachment means 5, which is attached to a momenttransducer 6, which is measuring the moment round the horizontal axis,point 7, situated along the centre line 8 - 8. Further, the momenttransducer 6 is attached to one end of a force transducer 9, which ismade only to measure forces in the direction of or parallel to thecentre line 8-8. The other end of the transducer 9 is attached to theground 10.

[0008] The inlet velocity of the material to the plate 3 is v₁ and theoutlet velocity is v₂. The height h₁ is the height of the sectional areaof the incoming material and h₂ the height of the outlet area. Toillustrate how a continues flow behave when sliding with constantgravitation along the tilted plate, the mass volume with the length l₁is shown at the inlet point and that will correspond to the volumeV=h₁·l₁·b, where b is the width of the measuring plate. At the outletpoint the same volume changes its dimensions to V=h₂·l₂·b which area ismarked as two equal cross-sectioned areas in the figure. At the distancefrom the inlet to the outlet of the plate the velocity has changed fromv₁ to v₂. It can be shown that the relative change of the velocity(v₁−v₂)/(v₁+v₂) is inversely proportional to the relative change of theheight of the material on the weighing plate (h₁−h₂)/(h₁+h₂). Thevelocity can be deduced by determining the heights of the material. Themoment (M) round the axis 7 is equal to the difference of weights of thematerial on each side of the centre line 8 - 8. On FIG. 1 it is seen asthe difference of material areas because of the heights h₁ and h₂, whichdifference h₁−h₂ is proportional to the moment measured by thetransducer 6. The sum h₁+h₂ is given from the measurement of the forcetransducer 9. As it is independent of the centre of gravity of the load,the measurement is the weight (G) which corresponds to the mean value(h₁+h₂)/2. It can be shown that the material flow (q) is equal to

q=G{square root}{square root over (C*G/M)}

[0009] where C is a constant and G/M represents the velocity.

[0010]FIG. 2 shows a simplified embodiment of a construction of thetransducers. The central part of the inclined plate 3 with side frame 11is shown. To the attachment means 5 a combined moment- and forcetransducer 12 is placed, which is attached to the ground 10. The upperpart of the combined transducer is the moment measuring transducer 13.This consists of a pair of recesses and the remaining central part isforming a cantilever 14. On the vertical sides a pair of strain gauges15+ and 15− are placed. A counter clock wise moment M will cause anincreasing strain in 15+ and a compression in 15−, which in known way ina bridge connection will produce a signal proportional to the moment M.The vertical load G will produce the same compression in both gauges andthe bridge signal is nil. The lower, horizontal part 16 of thetransducer is made with a rectangular hole 17 leaving two flat bars 18.The bars are provided with two pairs of strain gauges 19+ and 19−, whichfor the vertical load G will cause an increasing strain in the gauges19+ and a compression in the gauges 19−, whereby a bridge connection forall four gauges will produce a signal proportional to the load G. Themoment M will produce a horizontal force in the bars 18 which will evenout the bridge signal to nil. The signals then are processed accordingto the equation above to get the flow q.

[0011] The load sensing unit 6,9 may also be mounted perpendicular tothe measuring plate 3, and as the load transducer is measuring loadsonly along and parallel with its centre line, the output signal will bereduced with cos φ, where φ is the tilting angle shown in FIG. 1.

[0012]FIG. 3 shows a side elevation view of a flow meter embodyingprinciples of the present invention and adopted to smaller capacitiesthan for which the design of the transducers on FIG. 2 are suitable. Themoment measuring means consists of a combination of a lever withconstant length and a conventional strain gauge load beam. The measuringplate 3 is provided with side frames 11. Under the centre of plate 3 isa shaft 33 attached in the cross direction of plate 3 and the shaft isformed to journals on each side of the plate. These journals are pivotedin holes in the brackets 20, which are attached to a bar 21 (not seen inthe figure). At its centre the bar 21 is attached to a vertical loadmeasuring unit below. This unit consists of a parallel frame with twopillars 22, 22 a on whose flat ends a pair of springy strips 23 arefirmly attached. The pillar 22 is attached to a L-formed frame 24, whichis resting on the ground 10. On pillar 22 a console 25 is mounted, whichis carrying the attached load beam 26. This consists of conventionalstrain gauge beam type, and senses loads in the cross direction to itslength axis and is via a strip 27 carrying the load on pillar 22 a,whereby the strip 27 in its other end is fastened to the head 28. On thepillar 22 a also is a console 29 on which the load beam 30 is attached.This is of the same type as the beam 26. To its free end a strip 31 isfastened, which one also is attached to the head 32 on plate 3 at adirection perpendicular to the under side of the plate 3. The distance afrom the centre of the shaft 33 to the strip 31 is the lever for themoment measurement with load beam 30. The distance from the centre ofthe shaft 33 to the gravity centres for different loads on each side ofthe shaft 33, when material slides along the plate 3, will in thisapplication be longer then the lever to the load beam. Hereby the outputsignal from the load beam 30 is changed.

[0013] The relation concerning the positions of the measuring means,with upper position of the moment measuring mean to the force measuringmean below, is in this case the same as in previous examples. Inprinciple it is also possible to exchange the positions of the moment-and load measuring units even if the moment measuring unit, being thesmallest quantity, easier will be overloaded. The pivot part formed bythe hole in the bracket 20 introduces friction errors at the momentmeasuring, which can be avoided by replacing them to flexible springelements in form of a flat, short strip in vertical position on eachside of plate 3 at the same position as the pivoting journals.Alternatively the pivot is given the form of cross spring elementsconsisting of two short strips placed into each other perpendicularpositions.

[0014] The calculating means required to determine the flow value ispreferably consisting of a computer, where also additional programmedfunctions to secure maximum measuring accuracy is included, such askeeping the velocity signal locked during transient loads, and automaticzero setting when the flow is absent. A requisite for the latter is thatno noise from individual particles is present, which is secured byanalysing the flow pattern.

[0015] From the background art to this invention it is learned that thefrictional force between the sliding material and the surface of thetilted plate, which introduces measuring errors, can be compensated forby a separate force measuring means. That error can also be correctedfor by the apparatus according to the present invention. A third loadmeasuring means, applied above the moment measuring means (FIG. 2, part13) but under the tilting plate 3, is arranged so that the force alongthe direction only of the tilting plate is measured. This can bearranged by fastening a force transducer 16 between the attachment means5 and the plate 3.

1. Apparatus for weighing the mass flow of fluent material, especiallyof particulate type, by determining the forces exerted on asubstantially in the direction of the flow rectilinear and inclinedplate (3) on which the material (1) is sliding, characterised in thatthe measuring plate (3) is fastened at one central level (8 - 8) of theplate; that the fastening device includes one moment measuring means (6)measuring the moment round a horizontal axis (7) which is crossing thevertical axis (8 - 8) and which means (6) is attached to a forcemeasuring means (9) measuring the force directed in or parallel to thevertical axis (8 - 8); that the force measuring means (9) is resting onor attached to the ground (10); and that the material flow (1) isdetermined by a calculating means from the output signals from themoment measuring means (6) and the force/load measuring means (9). 2.Apparatus according to claim 1 , characterised in that the two measuringmeans (6), (9) are made in one solid piece/all of a piece (12). 3.Apparatus according to claim 1 or 2 , characterised in that the forcemeasuring means is fastened at one central level (8 - 8) of the plate(3); and that the moment measuring means is placed below said forcemeasuring means and is resting on or attached to the ground. 4.Apparatus according to claim 1 , characterised in that the calculatingmeans includes adapting circuits for the measuring means; and that amicroprocessor for handling the calculations is included.
 5. Apparatusaccording to claim 4 , characterised in that the calculating means isprovided by a function which at flow disturbances locks by time thesignal value from the moment measuring means (6).
 6. Apparatus accordingto claim 4 , characterised in that the calculating means is providedwith automatic zero setting functions acting when the flow is absent. 7.Apparatus according to claim 6 , characterised in that the calculatingmeans is provided with automatic zero setting functions acting when theflow is absent, that this is decided by continuously sensing said flowlevel by analysing a flow-dependent signal pattern from the transducers;and that then the transducer signals are kept to zero.
 8. Apparatusaccording to claim 1 , characterised in that the moment measuring meanscomprises a combination of a force transducer (30) and a lever (a); thatthe centre end of said lever is situated (positioned) at the centre of ashaft (33) which is formed to journals at each long side of the tiltingplate (3), said journals is pivoting round brackets (20) on each side ofsaid tilting plate and said brackets are attached to a verticalmeasuring means (22-28); that the other end of said lever is connectedto a force measuring means (30); and that the direction of said lever isarranged so that the attacking force from the measured momentcorresponds to the force measuring direction of said force measuringmeans.
 9. Apparatus according to claims 1 and 8, characterised in thatthe pivoting support consists of elastically supported bearings; andthat said bearings consist of vertically arranged strips on each longside of the tilting plate (3).
 10. Apparatus according to claims 1 and8, characterised in that the pivoting support consist of elasticallysupported bearings; and that said bearings consists of a pair of stripscrossways arranged on each long side of the tilting plate (3). 11.Apparatus according to claims 1 and 4, characterised in that between theunder side of the plate (3) and the moment measuring means (6) isincluded a force measuring means performed to measure a force only inopposite direction to that of the sliding material; and that in thecalculating means is included functions to calculate the interferencefrom said force on the value of the material flow.